Attempting to move Vertices in 2.80 creates new ones instead
Had a sudden flash of inspiration... there were multiple vertices on the same spot!

After selecting them all and doing S, Z, 0 I then selected what looked to be the old unmoved ones as well, and did S, Z, 0 again. And then added the 'unmoved' ones to the selection again and did S, Z, 0 yet again.

At that point, it became clear that that was the issue.



A couple more rounds, and the 'bump' is finally gone!



Wish I'd thought of that before bothering you all, but hopefully this'll help someone else in the same boat!